NK cells are cytotoxic lymphocytes that lyse certain tumor and virus infected cells without any prior stimulation or immunization. NK cells are also potent producers of various cytokines, e.g. IFN-y, TNF-α, GM-CSF and IL-3. Therefore, NK cells are also believed to function as regulatory cells in the immune system, influencing other cells and responses.
In humans, NK cells are broadly defined as CD56+CD3− lymphocytes. The cytotoxic activity of NK cells is tightly controlled by a balance between the activating and inhibitory signals from receptors on the cell surface. A main group of receptors that inhibits NK cell activation are the inhibitory killer immunoglobulin-like receptors (KIRs). Upon recognition of self MHC class I molecules on the target cells, these receptors deliver an inhibitory signal that stops the activating signalling cascade, keeping cells with normal MHC class I expression from NK cell lysis. Activating receptors include the natural cytotoxicity receptors (NCR) and NKG2D that push the balance towards cytolytic action through engagement with different ligands on the target cell surface. Thus, NK cell recognition of target cells is tightly regulated by processes involving the integration of signals delivered from multiple activating and inhibitory receptors.
Several strategies have been used to enhance NK cell responses to tumors. Cytokines are used in the treatment of some human cancers and NK cell differentiation and activation is affected by cytokines such as interleukins (e.g. IL-2, IL-12. IL-15, IL-18 and IL-21). The effect of IL-2 administration on activation and expansion of NK cells in cancer patients have been assessed in several trials with mixed outcomes depending on type of tumor and the conditions used for IL-2 administration. One example of cellular therapy is the NK cell-mediated killing of leukaemia cells which is based on NK cell alloreactivity.
Multiple myeloma (MM) is a plasma cell neoplasm characterized by the clonal proliferation of plasma cells in the bone marrow (BM). The malignant cells are associated with the synthesis of monoclonal immunoglobulin and a high incidence of osteolytic bone lesions. The disease accounts for about 2% of all cancer deaths and nearly 20% of deaths caused by hematological malignancies. Although allogeneic stem cell transplantation occasionally cures these patients and drugs like thalidomide, lenalidomide and bortezomib have improved outcome, high-dose chemotherapy followed by autologous stem cell transplantation (ASCT) still appears to be the best treatment for patients up to 65-70 years of age. However the great majority of patients with MM are incurable due to the persistence of minimal residual disease. Thus, novel methods for complementing or improving current treatments are needed.
In order to use NK cells in adoptive immunotherapeutic strategies, the availability of functionally active NK cells on a clinical scale is crucial. Clinical trials that have been performed using autologous NK cells were hampered by the fact that the cell dose was inadaptable to the demands of clinical trials. Therefore, the development of protocols for large-scale generation of NK cells is important to evaluate the potential of NK cell-based therapeutic protocols. Examples of reports that deal with ways of expanding and culturing human NK-cells are U.S. Ser. No. 10/242,788 and WO 2006/052534.
Human ex vivo expanded NK cells would be favorable candidates for immunotherapeutic approaches against malignant disease if they could target tumor cells.